1. Field of the Invention
The present invention relates to a data conversion system having a video interface function for serially transferring image data to, e.g., a printer or a display and, more particularly, to high-speed conversion from parallel data to serial data in an asynchronous data transfer mode in an image information apparatus or a document processing apparatus.
2. Description of the Related Art
A conventional image information equipment or document processing apparatus comprises an interface circuit for connection with a printer (printing device) or a CRT (display device). The interface circuit performs bit-width conversion since the data transfer rate or transfer bit width of an image data transfer bus of the equipment or apparatus is different from that of the printer or CRT, which receives image data.
In order to absorb a difference in transfer rate, a line buffer is used, and the write data width to this buffer is adjusted to the data width of the line buffer (data packing processing).
When a data width is converted to a 1-bit width, i.e., parallel data is converted into serial data in an asynchronous data transfer mode, data read out from a line buffer is written in a shift register, and is shifted in synchronism with a synchronization clock.
However, the packing (data width adjusting) processing for adjusting the write data width to the line buffer to the data width of the buffer requires a long period of time for writing data in the line buffer. For this reason, when the read frequency to the line buffer exceeds the write frequency, the buffer becomes empty during data transfer, and data transfer is disabled.